questionbase.50megs.com GCSE Revision Notes

Chemistry Revision Notes – Rates Of Reaction 1. The rate of reaction can be increased by:

• Increasing the surface area. • Increasing the temperature. • Increasing the concentration of the reactants. • Using a catalyst.

2. A catalyst lowers the activation energy of a reaction by providing a surface for particles to react on. A catalyst can also slow down reactions.

3. An endothermic reaction cools down (it takes in heat from the surroundings). 4. An exothermic reaction heats up (it gives out heat to the surroundings). 5. The most common example of an exothermic reaction is combustion. 6. Collision theory defines the rate of a reaction – the more that the reactants bump into each other,

the more likely they are to react. 7. Energy bond diagrams:

8. The enthalpy of an exothermic reaction is expressed by –∆H. 9. The enthalpy of an endothermic reaction is expressed by +∆H. 10. Energy is needed to break covalent bonds, i.e. breaking bonds is endothermic. 11. Energy is released when new covalent bonds are made, i.e. making bonds is exothermic. 12. In an exothermic reaction, the energy of the products is less than the energy of the reactants. 13. In an endothermic reaction, the energy of products is greater than the energy of the reactants. 14. Reversible reactions are expressed in the form A + B Á C + D, whereby the top arrow represents

the forward reaction, and the bottom arrow represents the backward reaction. 15. In equilibrium, the forward reaction matches the backward reaction. 16. Equilibrium can be moved to the right or the left by:

• Changing the concentration – this favours the forward reaction if the reactants are increased in concentration, and the backward reaction if the products are increased in concentration).

• Changing the temperature – this favours the endothermic reaction if increased, and the exothermic reaction if decreased.

• Changing the pressure – this favours the reaction that produces less volume if increased, and the reaction that produces more volume if decreased.

17. Adding a catalyst has no effect on the position of the equilibrium, but it allows equilibrium to be reached more quickly.